1. Field of the Invention
Medical examinations to identify and diagnose hearing defects have historically involved physical examination of the ear canal and observing the subjective response to sound stimulus. Generally, the subjective sensitivity to volume has been routinely measured at several auditory frequencies. Most recently, the evoked action potentials resulting from auditory stimulation and auditory brainstem responses have been measured in an effort to identify more specifically the cause and degree of hearing loss and other hearing defects. Auditory brainstem response measurements involve auditory stimulation and measurement of the magnitude and response time of electrical signals originating from the otic nerve and brainstem in response to the auditory stimulus. The electrical signals are detected by using non-invasive electrodes mounted on the skin surface.
2. Description of the Prior Art
Ear canal mounted devices for providing auditory stimuli to the ear are well known in the art. The sound is frequently transmitted to the ear through a tubular component as shown in U.S. Pat. No. Re. 26,174 which is directed to a hearing aid and a method for its construction. In general, these devices are designed to provide a close or snug fit in the ear canal and may include resilient components to achieve this. U.S. Pat. No. 3,783,201 discloses a hearing aid with a flexible construction. U.S. Pat. No. 4,133,984 describes a hearing aid with a flexible, expandable (inflatable) end portion. These hearing aids are designed to provide auditory stimulus but are not suitable or intended to receive electrical signals from the surface of the ear canal.
Electrodes suitable for use in conducting electrical signals from the surface of the skin are commonly used in electrocardiology measurements and the like. These electrodes are designed to have a low resistance and high sensitivity as described in U.S. Pat. Nos. 3,547,104 and 4,166,453. The latter patent describes a body electrode comprising a porous foam disc impregnated with an electrically conductive gel and other components cooperating therewith made of electrically conductive plastic. These electrodes are designed to be easily applied to a flat skin surface. They are not suitable for ear canal insertion and are not adapted for providing any sound stimulus.
Conductive metal electrodes which penetrate the eardrum, i.e. transtympanic electrodes, have been used. They require use of an anesthetic, are painful and often result in infection. They have been used to pick up signals from close to the cochlea for increased signal strength of the first auditory action wave.
Typical electrodes designed for non-invasive external ear canal insertion for measuring auditory brainstem responses are described in Scandanavian Audiology Supplementum No. 13, titled "Scandanavian Symposium on Brain Stem" edited by T. Lundborg (Apr. 7-8, 1981). "Identification of Wave I by Means of Atraumatic Ear Canal Electrode" by Walter et al, pp 63-64 describes an electrode made of silver wire, the terminal end supporting a salt water impregnated cotton ball. "An Improved Technique for the Non-Invasive Recording of Brain-Stem Responses with a Specially Constructed Meatal Electrode" by Lang et al, pp 59-62 describe a silver wire electrode designed to be worn under earphones, the terminal end to be coated with electrode jelly. "Ear-canal Compared to Mastoid Electrode Placement in BRA" by Harder et al, pp 55-57 describes an acrylic plastic electrode with a silver element having a silver chloride surface layer embedded therein. Electrode paste is applied to the ear canal before inserting the electrode. These devices do not have the construction of the ear electrodes of this invention and do not provide the low resistance and high sensitivity thereof. Also, these devices have no means for providing auditory stimuli. In each referenced system, the auditory stimuli is provided through devices not physically associated with the ear electrode. An even greater limitation of all of the invasive and non-invasive electrodes of the prior art has been the requirement that a physician make the insertion because of the potential for inadvertent penetration of the eardrum.